This improvement relates to a method and means for improving character clarity and character density in a cathode ray tube display. The frequency response characteristics of CRT video displays significantly affect the price of the display; cost increases significantly with increases in the quality of the frequency response. State-of-the-art cathode ray tube displays are frequently limited to the range of 40 to 48 characters per line.
There are frequency restrictions associated with state-of-the-art cathode ray tubes for alphanumeric character displays. The quality of alphanumeric character displays on a television monitor is restricted by the maximum Z-modulation frequency at which the video response falls below 3 dB, as well as being restricted by the change in video response for a given change in the modulation frequency.
For point-to-point unblanking with maximum character density per line, the system must frequently be operated at the point which creates the greatest response change for a given frequency change. For instance, the upper bar of a "T" will appear brighter than the adjacent vertical components making up the remainder of the letter. The use of a dot (discrete charater element) matrix unblanked display permits use of the same fundamental frequency for all horizontal and vertical segments of characters, but this fundamental frequency is often 5 to 14 times the lowest fundamental frequency composing a specific character.
Typical state-of-the-art video monitors utilize a horizontal sweep frequency of approximately 15.75 KHz and exhibit an 8 MHz Z-modulation cut-off frequency. Due to retrace time and inherent linearity restrictions, only 66 to 75 percent of the sweep is available for displaying alphanumeric characters and symbols. In terms of sweep time, this results in approximately 42 to 48 microseconds available for Z-modulation unblanking.
Typical dot matrices are 5, 7, or 9 spaces wide and 7, 8, or 9 spaces high. Character density increases inversely with matrix size while character definition increases directly with matrix size. In order to properly display a 64 EBCDIC or ASCII character set, a matrix no less than 7 spaces wide must be chosen. A 5 space wide matrix results in a poorly defined 48 character set.
It is, therefore, an object of the present invention to provide a method and means to improve both the clarity and density of characters displayed on a CRT.
It is another object to improve the appearance of the characters displayed.